Stars expand as they grow old. As their core runs out of hydrogen and then helium, the core contacts and the outer layers expand, cool, and become less bright. This is a red giant or a red super giant (depending on the initial mass of the star). It will eventualy collapse and explode. Its fate is determined by the original mass of the star; it will become either a black dwarf, neutron star, or black hole.

The Death of Small Stars (with a mass up to 1 1/2 times that of the Sun): Black Dwarfs


After expanding and reaching the enormous red giant phase, the outer layers of the star continue to expand. As this happens, the core contracts; the helium atoms in the core fuse together, forming carbon atoms and releasing energy. The core is now stable since the carbon atoms are not further compressable.

The Pistol nebula: a planetary nebula in Sagittarius.

The Egg nebula: a planetary nebula formed a few hundred years ago.

Now the outer layers of the star start to drift off into space, forming a planetary nebula (a planetary nebula has nothing to do with planets).

The star loses most of its mass to the nebula. The star cools and shrinks; it will eventually be only a few thousand miles in diameter!

A White dwarf star: (circled) in the globular cluster M4.

The star is now a white dwarf, a stable star with no nuclear fuel. It radiates its left-over heat for billions of years. When its heat is all dispersed, it will be a cold, dark black dwarf - essentially a dead star (perhaps replete with diamonds, highly compressed carbon).

NOVA
A nova is a white dwarf star that suddenly increases in brightness by several magnitudes. It fades very slowly.

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The Death of Huge Stars (over 1.5 times the mass of the Sun): Supernovae and ...

Betelguese, a red supergiant in Orion.

When huge stars grow old, they become even more enormous red supergiants (as their core fuses all the hydrogen into helium). Their core shrinks, becoming hotter and denser. With these changes, different nuclear processes occur; fusion now produces heavier elements (this temporarily stop the core's shrinking).

Eventually this core collapses (in an instant). As the iron atoms are crushed together in this gravitational collapse, the core temperature rises to about 100 billion degrees.

Supernova N132D: 3,000 years after a supernova, ejecting stellar material (including oxygen-rich gas) in luminescent shock fronts. It is located in the Large Magellanic Cloud (169,000 light-years from Earth).

Supernova SN1987A: the beginning of a supernova.

The repulsive electrical forces between the atoms' nuclei overcomes the gravitational forces, causing a massive, bright, short-lived explosion called a supernova. During the explosion, shock waves, blow away the star's outer layers.

The next stage depends on the star's remaining mass:

The Crab nebula: the remnant of a supernova in A.D. 1054. It has a rapidly spinning neutron star.

• If the star's remaining mass is between 1 1/2 to 3 times the mass of the Sun, it will collapse into a small, dense neutron star (about ten miles in diameter, about 1.4 times the mass of the Sun, with an extraordinarily strong magnetic field, and rapid spin).
• If the star's remaining mass is greater than three times the mass of the Sun, the star contracts tremendously and becomes a black hole (incredibly dense with a gravitational field so strong that even light cannot escape).

EVOLVED STAR
An evolved star is an old star that is near the end of its existence. Its nuclear fuel is mostly gone. The star loses mass from its surface, producing a stellar wind (gas that is ejected from the surface of a star).

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